1. Field of the Invention
The present invention relates in general to a pneumatic tire and more particularly to a pneumatic tire which is designed to provide not only good controllability, stability and wear resistability particularly in summer but also improved traction and braking abilities on snow or iced road surface in winter.
2. Description of the Prior Art
In order to improve the traction and braking abilities and controllability (hereinafter, which will be refered to "snow gripping ability") of tires to snow or iced road surface, spiked tires have been hitherto proposed and practically put in use. However, wear of the spikes of the tires and wear of the road surfaces resulting from scratching by the spikes have caused a severe social problem, viz., the dust pollution.
In the meantime, in the tires of a studless type which has no spike pins mounted on the tires, various attempts have been made to the shape of the tread pattern and the material of the tread for the purpose of improving the snow gripping ability of the tire. However, it has been impossible to increase the snow gripping ability to the level of the spiked tire. In fact, using a low glass transition point polymer as a material of the tread and using a low melting point softening agent as an additive of the rubber composition have been taken into consideration for assuring satisfactory friction between the tread and the road surface at low temperature. However, this method brings about a deterioration of the snow gripping ability of the tread.
Furthermore, tires which use foamed rubber as a material of the tread have been proposed by Japanese Patent Second Provisional Publication 40-4641, U.S. Pat. No. 4,249,588 and Japanese Patent Second Provisional Publication No. 56-154304. In case of the 40-4641 Publication, synthetic rubbers, such as high styrene-rubber, of the type which has a considerable hysteresis loss are used for the tread. However, using such rubber components increases the glass transition point of the tread rubber and thus increases the hardness of the same at a lower temperature thereby deteriorating the snow gripping ability of the tread. In case of the '588 Publication, a rubber having a compressibility in the range of about 1 to 800 psi at 50% compression at 25.degree. C. is used as a material of the tread. However, it is generally said that a desired control responsibility of the tread is obtained only when the tread has at least 400 psi. Furthermore, in case of the 56-154304 Publication, foamed rubber having a considerable hardness is used in place of solid rubber for the purpose of reducing the weight of the tire. However, as is known, satisfactory snow gripping ability can not be obtained from such hardened tread.
In addition to the above, an attempt of mixing regular additives, such as sand, carborundum grain and metal grain, to the rubber composition for the tread has been hitherto made for the purpose of improving the snow gripping ability. However, this attempt has brought about deterioration of the wear resisting ability of the tread.
Furthermore, an attempt at reducing the hardness of the rubber of the tread has been also made for the purpose of improving the braking ability of the tread on iced road surface. That is, in order to achieve this, reduction of reinforcing agents, reduction of vulcanization density and increase of softener and oil have been employed. However, as is well known to those skilled in the art, the reduction of the reinforcing agents brings about a deterioration of the braking ability of the tread on wet road surface, the reduction of the vulcanization density brings about a considerable permanent set of the rubber and the increase of the amounts of softener and oil brings about a great change in hardness of the rubber during use of the same.